Steel material for line pipes, method for producing the same, and method for producing line pipe

ABSTRACT

A method for producing a steel material for line pipes including heating a steel having a specific composition to a temperature of 1000° C. to 1200° C.; performing hot rolling such that a cumulative rolling reduction ratio in a non-recrystallization temperature range is 60% or more, a cumulative rolling reduction ratio in a temperature range of (a rolling finish temperature +20° C.) or less is 50% or more, and a rolling finish temperature is the Ar 3  transformation point or more and 790° C. or less; subsequently performing accelerated cooling from a temperature of the Ar 3  transformation point or more, at a cooling rate of 10° C./s or more, to a cooling stop temperature of 200° C. to 450° C.; and then performing reheating such that the temperature of a surface of the steel plate is 350° C. to 550° C. and the temperature of the center of the steel plate is less than 550° C.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2019/001854, filedJan. 22, 2019, which claims priority to Japanese Patent Application No.2018-013320, filed Jan. 30, 2018, the disclosures of these applicationsbeing incorporated herein by reference in their entireties for allpurposes.

FIELD OF THE INVENTION

The present invention relates to a steel material for line pipes, amethod for producing the steel material for line pipes, and a method forproducing a line pipe. The present invention relates to a steel materialfor line pipes which is suitable as a material for line pipes used forthe transportation of oil and natural gas and is particularly suitableas a material for offshore pipelines, which are required to have a highcollapse resistant performance, a method for producing such a steelmaterial for line pipes, and a method for producing a line pipe. Theterm “compressive strength” used herein refers to 0.5% compressive proofstrength and is also referred to as “compressive yield strength”, unlessotherwise specified.

BACKGROUND OF THE INVENTION

With an increasing demand for energy, the development of oil and naturalgas pipelines has been active. Various pipelines that extend across seahave been developed in order to cope with a situation where gas fieldsor oil fields are located at remoter places or versatility in transportroutes. Line pipes used as offshore pipelines have a larger wallthickness than onshore pipelines in order to prevent collapse due towater pressure. Furthermore, the line pipes used as offshore pipelinesare required to have a high degree of roundness. In addition, as for theproperties of line pipes, the line pipes need to have a high compressivestrength in order to resist the compression stress caused in thecircumferential direction of the pipes by an external pressure.

Since the final step of a method for making UOE steel pipes includes apipe expanding process, the pipes are compressed after the pipes havebeen subjected to a tensile deformation in the circumferential directionof the pipes. Consequently, compressive yield strength may be reduceddue to the Bauschinger effect.

There have been various studies of improvement of the collapse resistantperformance of UOE steel pipes. Patent Literature 1 discloses a methodin which a steel pipe is heated by Joule heating and expanded, and thetemperature is subsequently held for a certain period of time or more.

As a method in which heating is performed subsequent to the pipeexpansion in order to restore the reduction in compressive yieldstrength caused by the Bauschinger effect as described above, PatentLiterature 2 proposes a method in which the outer surface of a steelpipe is heated to a temperature higher than that of the inner surface inorder to restore the impact due to the Bauschinger effect caused in theouter surface-side portion of the steel pipe which has been subjected toa tensile deformation and to maintain the strain hardening of the innersurface-side portion due to compression. Patent Literature 3 proposes amethod in which, in a steel plate making process using a steelcontaining Nb and Ti, accelerated cooling is performed from atemperature of the Ar₃ transformation point or more to the temperatureof 300° C. or less subsequent to hot rolling and heating is performedafter a steel pipe has been formed by the UOE process.

On the other hand, as a method in which the compressive strength of asteel pipe is increased by adjusting the conditions under which thesteel pipe is formed, instead of performing heating subsequent to thepipe expansion, Patent Literature 4 discloses a method in which thecompression ratio at which compression is performed when a steel pipe isformed using the O-ing press is set to be higher than the expansionratio at which pipe expansion is performed in the subsequent step.

Patent Literature 5 discloses a method in which the diameter of a steelpipe which passes through the vicinity of a weld zone, which has a lowcompressive strength, and the position that forms an angle of 180° withrespect to the weld zone is set to be the maximum diameter of the steelpipe in order to enhance the collapse resistant performance of the steelpipe.

Patent Literature 6 proposes a steel plate capable of limiting areduction in yield stress due to the Bauschinger effect, which isproduced by performing reheating subsequent to accelerated cooling toreduce the fraction of the hard second phase in the surface-layerportion of the steel plate.

Patent Literature 7 proposes a method for producing a high-strengthsteel plate for line pipes for sour gas service having a thickness of 30mm or more, in which the surface-layer portion of a steel plate isheated in a reheating process performed subsequent to acceleratedcooling while a rise in the temperature of the center of the steel plateis suppressed.

PATENT LITERATURE

PTL 1: Japanese Unexamined Patent Application Publication

PTL 2: Japanese Unexamined Patent Application Publication No.2003-342639

PTL 3: Japanese Unexamined Patent Application Publication No. 2004-35925

PTL 4: Japanese Unexamined Patent Application Publication No.2002-102931

PTL 5: Japanese Unexamined Patent Application Publication No.2003-340519

PTL 6: Japanese Unexamined Patent Application Publication No. 2008-56962

PTL 7: Japanese Unexamined Patent Application Publication No. 2009-52137

SUMMARY OF THE INVENTION

According to the method described in Patent Literature 1, dislocationbrought about by the pipe expansion is eliminated or dispersed and,consequently, compressive strength is increased. However, this methodrequires the Joule heating to be continued for five minutes or moresubsequent to the pipe expansion and is therefore poor in terms ofproductivity.

In the method described in Patent Literature 2, it is necessary toindividually manage the temperatures at which the outer and innersurfaces of a steel pipe are heated and the amounts of time during whichthe outer and inner surfaces of the steel pipe are heated. This isdifficult in terms of the actual manufacture. It is considerablydifficult to manage the quality of steel pipes in a mass productionprocess. The method described in Patent Literature 3 requires theaccelerated cooling stop temperature in the production of a steel plateto be a low temperature of 300° C. or less. This may increase thedistortion of a steel plate and degrades the roundness of a steel pipeproduced by the UOE process. Furthermore, since the accelerated coolingis performed from a temperature of the Ar_(a) point or more, it isnecessary to perform rolling at a relatively high temperature. This mayresult in the degradation of toughness.

According to the method described in Patent Literature 4, tensilepre-strain substantially does not occur in the circumferential directionof the pipe. Accordingly, the Bauschinger effect is not produced and ahigh compressive strength may be achieved. However, a low expansionratio makes it difficult to maintain the roundness of a steel pipe andmay degrade the collapse resistant performance of the steel pipe.

The portion of a pipeline which is prone to collapse when the pipelineis actually constructed is a portion (sag-bend portion) subjected to abending deformation when the pipe reaches the sea-bed. When a pipelineis constructed, girth welding is performed on the pipe and the pipes arelaid on the seabed without reference to the positions of weld zones ofsteel pipes. Therefore, even if steel pipes are produced by performingpipe forming and welding such that a cross section of each of the steelpipes has the maximum diameter at the seam weld zone as described inPatent Literature 5, it is not possible to determine the positions ofthe seam weld zones when a pipeline is constructed actually. Thus, thetechnology according to Patent Literature 5 does not produce virtuallyany advantageous effects.

The steel plate described in Patent Literature 6 needs to be heated inthe reheating until the center of the steel plate is heated. This mayresult in the degradation of a DWTT (drop weight tear test) property.Therefore, it is difficult to use this steel plate for producingdeep-sea thick-walled line pipes. In addition, the steel plate has roomfor improvement in terms of increase in the thickness of the steelplate.

According to the method described in Patent Literature 7, the fractionof the hard second phase in the surface-layer portion of a steel platemay be reduced while the degradation of a DWTT (drop weight tear test)property is limited. This may reduce the hardness of a surface-layerportion and inconsistencies in the material property of the steel plate.Furthermore, the reduction in the fraction of the hard second phase mayreduce the Bauschinger effect. However, it is difficult to consistentlyachieve a strength of X70 grade or more while maintaining a DWTTproperty by the technology described in Patent Literature 7.

Aspects of the present invention was made in view of the above-describedcircumstances. An object according to aspects of the present inventionis to provide a steel material for line pipes having a heavy wallthickness of 30 mm or more, a high strength required for applying thesteel material to offshore pipelines, excellent low-temperaturetoughness, and an excellent DWTT property, a method for producing thesteel material for line pipes, and a method for producing a line pipe.

The inventors of the present invention conducted extensive studies inorder to limit the reduction in compressive strength due to theBauschinger effect and maintain strength and toughness and, as a result,found the following facts.

(a) The reduction in compressive strength due to the Bauschinger effectis induced by the back stress caused as a result of the dislocationaccumulation at the interfaces between different phases and the hardsecond phase. For preventing this, first, it is effective to form auniform microstructure in order to reduce the interfaces between thesoft and hard phases, at which dislocations are integrated. Accordingly,forming a metal microstructure composed primarily of bainite in whichthe formation of soft polygonal ferrite and a hard martensite-austeniteconstituent is suppressed may limit the reduction in compressivestrength due to the Bauschinger effect.

(b) It is difficult to completely inhibit the formation of themartensite-austenite constituent (hereinafter, may be referred to simplyas “MA”) in high-strength steel produced by accelerated cooling and, inparticular, thick-walled steel plates used for producing offshorepipelines because such high-strength steel and thick-walled steel plateshave high hardenability as a result of containing large amounts ofalloying elements to achieve an intended strength. However, thereduction in compressive strength due to the Bauschinger effect may belimited when MA is decomposed into cementite by, for example, performingreheating subsequent to accelerated cooling. Although performingreheating subsequent to accelerated cooling may reduce strength, arequired strength may be achieved by controlling the reheatingtemperature to fall within a predetermined temperature range. Reheatingenables a high compressive strength relative to tensile strength to beachieved. Furthermore, the hardness of the surface layer may be reduced.Thus, a steel pipe having suitable roundness may be produced stably.

(c) For enhancing low-temperature toughness, it is effective to reducethe size of microstructures by lowering the rolling temperature at whicha steel plate is hot-rolled. However, if the rolling temperature isexcessively low, polygonal ferrite may be formed and a mixedmicrostructure including bainite and polygonal ferrite may be formedsubsequent to the accelerated cooling. This increases the Bauschingereffect. On the other hand, adjusting the composition of the steelcontributes to reduction in formation of polygonal ferrite being formedsubsequent to rolling with a low rolling temperature . This enables bothsuitable low-temperature toughness and suitable compressive strength tobe achieved. In addition, controlling the rolling reduction during hotrolling enables introduction of a number of deformation bands, whichserve as nuclei for transformation, and refinement of microstructures.This enables even a thick-walled steel plate having a thickness of 30 mmor more to have high low-temperature toughness.

Aspects of the present invention were made on the basis of the abovefindings and additional studies. The summary of aspects of the presentinvention is as follows.

[1] A method for producing a steel material for line pipes, the steelmaterial having a tensile strength of 570 MPa or more, a compressivestrength of 440 MPa or more, and a thickness of 30 mm or more, themethod including heating a steel having a composition containing, bymass,

C: 0.030% to 0.10%,

Si: 0.01% to 0.30%,

Mn: 1.0% to 2.0%,

Nb: 0.005% to 0.050%,

Ti: 0.005% to 0.025%, and

Al: 0.08% or less,

the composition further containing one or more elements selected from,by mass,

Cu: 0.5% or less,

Ni: 1.0% or less,

Cr: 1.0% or less,

Mo: 0.5% or less, and

V: 0.1% or less,

wherein a Ceq value represented by Formula (1) is 0.350 or more, a Pcmvalue represented by Formula (2) is 0.20 or less, and an Ar_(a)transformation point represented by Formula (3) is 750° C. or less, withthe balance being Fe and inevitable impurities, to a temperature of1000° C. to 1200° C.; performing hot rolling such that a cumulativerolling reduction ratio in a non-recrystallization temperature range is60% or more, such that a cumulative rolling reduction ratio in atemperature range of (a rolling finish temperature +20° C.) or less is50% or more, and such that a rolling finish temperature is the Ar₃transformation point or more and 790° C. or less, the rolling finishtemperature being an average temperature of a steel plate; subsequentlyperforming accelerated cooling from a temperature of the Ar₃transformation point or more, at a cooling rate of 10° C./s or more, toa cooling stop temperature of 200° C. to 450° C., the cooling stoptemperature being an average temperature of the steel plate; and thenperforming reheating such that the temperature of a surface of the steelplate is 350° C. to 550° C. and such that the temperature of the centerof the steel plate is less than 550° C.,

Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5   (1)

Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10   (2)

Ar₃(° C.)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo   (3)

wherein, in Formulae (1) to (3), the symbol of each element representsthe content (mass %) of the element and is zero when the compositiondoes not contain the element.

[2] A method for producing a line pipe having a tensile strength of 570MPa or more, a compressive strength of 440 MPa or more, and a thicknessof 30 mm or more, the method including cold forming a steel material forline pipes produced by the method described in [1] into a steelpipe-like shape; joining butting edges to each other by seam welding;and subsequently performing pipe expansion at an expansion ratio of 1.2%or less to produce a steel pipe.

[3] A steel material for line pipes, the steel material having a tensilestrength of 570 MPa or more, a compressive strength of 440 MPa or more,and a thickness of 30 mm or more, the steel material including acomposition containing, by mass,

C: 0.030% to 0.10%,

Si: 0.01% to 0.30%,

Mn: 1.0% to 2.0%,

Nb: 0.005% to 0.050%,

Ti: 0.005% to 0.025%, and

Al: 0.08% or less,

the composition further containing one or more elements selected from,by mass,

Cu: 0.5% or less,

Ni: 1.0% or less,

Cr: 1.0% or less,

Mo: 0.5% or less, and

V: 0.1% or less,

wherein a Ceq value represented by Formula (1) is 0.350 or more, a Pcmvalue represented by Formula (2) is 0.20 or less, and a Ar₃transformation point represented by Formula (3) is 750° C. or less, withthe balance being Fe and inevitable impurities,

the steel material further including a metal microstructure composedprimarily of bainite, wherein an area fraction of polygonal ferrite at aposition of ¼ plate thickness is 10% or less, an area fraction ofmartensite-austenite constituent at the position of ¼ plate thickness is5% or less, and an average grain size of bainite at a position of ½plate thickness is 10 μm or less,

Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5   (1)

Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10   (2)

Ar₃(° C.)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo   (3)

wherein, in Formulae (1) to (3), the symbol of each element representsthe content (mass %) of the element and is zero when the compositiondoes not contain the element.

[4] The steel material for line pipes described in [3], wherein a ratioof compressive strength to tensile strength is 0.748 or more, andwherein a hardness measured at a position 1.5 mm from an inner surfaceof a steel pipe is HV 260 or less.

[5] A method for producing a line pipe having a tensile strength of 570MPa or more, a compressive strength of 440 MPa or more, and a thicknessof 30 mm or more, the method including cold forming a steel material forline pipes described in [3] or [4] into a steel pipe-like shape; joiningbutting edges to each other by seam welding; and subsequently performingpipe expansion at an expansion ratio of 1.2% or less to produce a steelpipe.

According to aspects of the present invention, a steel material for linepipes which has a high strength, excellent low-temperature toughness,and an excellent DWTT property may be produced. Aspects of the presentinvention may be suitably applied to offshore pipelines.

According to aspects of the present invention, a thick-walled line pipehaving excellent low-temperature toughness and a high compressivestrength may be provided without employing special conditions forforming steel pipes or performing a heat treatment subsequent to pipemaking.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

An embodiment of the present invention is described below. Whenreferring to the contents of constituent elements, the symbol “%” refersto “% by mass” unless otherwise specified.

1. Chemical Composition C: 0.030% to 0.10%

C is an element most effective in increasing the strength of a steelplate produced by accelerated cooling. However, if the C content is lessthan 0.030%, a sufficiently high strength may not be maintained. On theother hand, if the C content is more than 0.10%, toughness may becomedegraded. In addition, the formation of MA may be accelerated. Thisresults in a reduction in compressive strength. Accordingly, the Ccontent is limited to 0.030% to 0.10%. Preferable lower limit of Ccontent is 0.040% and preferable upper limit is 0.098%.

Si: 0.01% to 0.30%

Si is contained for deoxidization. However, if the Si content is lessthan 0.01%, a sufficient deoxidation effect may not be achieved. On theother hand, if the Si content is more than 0.30%, toughness may becomedegraded. In addition, the formation of MA may be accelerated. Thisresults in a reduction in compressive strength. Accordingly, the Sicontent is limited to 0.01% to 0.30%. Preferable lower limit of Sicontent is 0.03% and preferable upper limit is 0.25%.

Mn: 1.0% to 2.0%

Mn: 1.0% to 2.0%. Mn is contained for increasing strength and enhancingtoughness. However, if the Mn content is less than 1.0%, the aboveadvantageous effects may not be produced to a sufficient degree. On theother hand, if the Mn content is more than 2.0%, toughness may becomedegraded. Accordingly, the Mn content is limited to 1.0% to 2.0%.Preferable lower limit of Mn content is 1.5% and preferable upper limitis 1.95%.

Nb: 0.005% to 0.050%

Nb reduces the size of microstructures and thereby enhances toughness.Nb also causes the formation of carbides, which increase strength.However, if the Nb content is less than 0.005%, the above advantageouseffects may not be produced to a sufficient degree. On the other hand,if the Nb content is more than 0.050%, the toughness of a weldheat-affected zone may become degraded. Accordingly, the Nb content islimited to 0.005% to 0.050%. Preferable lower limit of Nb content is0.010% and preferable upper limit is 0.040%.

Ti: 0.005% to 0.025%

Ti suppresses coarsening of austenite grains during heating of slabs bythe pinning effect of TiN and thereby enhances toughness. However, ifthe Ti content is less than 0.005%, the above advantageous effects maynot be produced to a sufficient degree. On the other hand, if the Ticontent is more than 0.025%, toughness may become degraded. Accordingly,the Ti content is limited to 0.005% to 0.025%. Preferable lower limit ofTi content is 0.008% and preferable upper limit is 0.023%.

Al: 0.08% or Less

Al is contained as a deoxidizing agent. However, if the Al content ismore than 0.08%, the cleanliness of steel may become degraded andtoughness may become degraded. Accordingly, the Al content is limited to0.08% or less. The Al content is preferably 0.05% or less.

In accordance with aspects of the present invention, one or moreelements selected from Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.0% orless, Mo: 0.5% or less, and V: 0.1% or less are contained.

Cu: 0.5% or Less

Cu is an element effective in improving toughness and increasingstrength. However, if the Cu content is more than 0.5%, the HAZtoughness of a weld zone may become degraded. Accordingly, in the casewhere Cu is contained, the Cu content is limited to 0.5% or less. Thelower limit for the Cu content is not specified. In the case where Cu iscontained, the Cu content is preferably 0.01% or more.

Ni: 1.0% or Less

Ni is an element effective in improving toughness and increasingstrength. However, if the Ni content is more than 1.0%, the HAZtoughness of a weld zone may become degraded. Accordingly, in the casewhere Ni is contained, the Ni content is limited to 1.0% or less. Thelower limit for the Ni content is not specified. In the case where Ni iscontained, the Ni content is preferably 0.01% or more.

Cr: 1.0% or Less

Cr is an element that enhances hardenability and thereby effectivelyincrease strength. However, if the Cr content is more than 1.0%, the HAZtoughness of a weld zone may become degraded. Accordingly, in the casewhere Cr is contained, the Cr content is limited to 1.0% or less. Thelower limit for the Cr content is not specified. In the case where Cr iscontained, the Cr content is preferably 0.01% or more.

Mo: 0.5% or Less

Mo is an element effective in improving toughness and increasingstrength. However, if the Mo content is more than 0.5%, the HAZtoughness of a weld zone may become degraded. Accordingly, in the casewhere Mo is contained, the Mo content is limited to 0.5% or less. Thelower limit for the Mo content is not specified. In the case where Mo iscontained, the Mo content is preferably 0.01% or more.

V: 0.1% or Less

V is an element that forms complex carbides as well as Nb and Ti and ismarkedly effective in increasing strength by precipitationstrengthening. However, if the V content is more than 0.1%, the HAZtoughness of a weld zone may become degraded. Accordingly, in the casewhere V is contained, the V content is limited to 0.1% or less. Thelower limit for the V content is not specified. In the case where V iscontained, the V content is preferably 0.01% or more.

In accordance with aspects of the present invention, the Ceq valuerepresented by Formula (1) is 0.350 or more, the Pcm value representedby Formula (2) is 0.20 or less, and the Ar_(a) transformation pointrepresented by Formula (3) is 750° C. or less.

Ceq Value: 0.350 or More

The Ceq value is limited to 0.350 or more. The Ceq value is representedby Formula (1) below. The Ceq value has a correlation with the strengthof base metal and is used as a measure of strength. If the Ceq value isless than 0.350, a high tensile strength of 570 MPa or more may not beachieved. Accordingly, the Ceq value is limited to 0.350 or more. TheCeq value is preferably 0.360 or more.

Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5   (1)

In Formula (1), the symbol of each element represents the content (mass%) of the element and is zero when the composition does not contain theelement.

Pcm Value: 0.20 or Less

The Pcm value is limited to 0.20 or less. The Pcm value is representedby Formula (2) below. The Pcm value is used as a measure of weldability;the higher the Pcm value, the lower the toughness of a welded HAZ. ThePcm value needs to be strictly limited particularly in a thick-walledhigh-strength steel, because the impact of the Pcm value is significantin the thick-walled high-strength steel. Accordingly, the Pcm value islimited to 0.20 or less. The Pcm value is preferably 0.19 or less.

Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10   (2)

In Formula (2), the symbol of each element represents the content (mass%) of the element and is zero when the composition does not contain theelement.

Ar₃ Transformation Point: 750° C. or Less

The Ar₃ transformation point is limited to 750° C. or less. Formula (3)below represents the Ar₃ transformation point. The higher the Ar₃transformation point, the higher the temperature at which ferrite isformed and the more the difficulty in achieving the metal microstructureaccording to aspects of the present invention. In addition, it becomesmore difficult to achieve both intended compressive strength andintended toughness. Accordingly, the composition is controlled such thatthe Ar₃ transformation point is 750° C. or less.

Ar₃(° C.)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo   (3)

In Formula (3), the symbol of each element represents the content (mass%) of the element and is zero when the composition does not contain theelement.

The remaining part of the composition which is other than theabove-described constituents, that is, the balance, includes Fe andinevitable impurities. The composition may contain an element other thanthe above-described elements such that the action and advantageouseffects according to aspects of the present invention are not impaired.

2. Metal Microstructure Composed Primarily of Bainite

The metal microstructure according to aspects of the present inventionis composed primarily of bainite in order to limit the reduction incompressive strength due to the Bauschinger effect. The expression “themetal microstructure according to aspects of the present invention iscomposed primarily of bainite” means that the area fraction of bainitein the entire metal microstructure is 85% or more. For limiting thereduction in compressive strength due to the Bauschinger effect, themetal microstructure is desirably composed only of bainite in order toprevent the dislocation accumulation at the interfaces between differentphases and the hard second phase. When the fraction of the balancemicrostructures other than bainite is 15% or less, they may beacceptable. Note that, the area fraction of bainite is measured at aposition of ¼ plate thickness.

Area Fractions of Polygonal Ferrite and Martensite-Austenite Constituentat Position of ¼ Plate Thickness Are 10% or Less and 5% or Less,Respectively

For reducing the Bauschinger effect and achieving a high compressivestrength, it is desirable to form a uniform microstructure free of asoft polygonal ferrite phase or a hard martensite-austenite constituentin order to reduce the likelihood of dislocations being locallyintegrated inside the microstructure during deformation. Accordingly, inaddition to forming a microstructure composed primarily of bainite asdescribed above, the area fractions of polygonal ferrite and themartensite-austenite constituent at a position of ¼ plate thickness arelimited to 10% or less and 5% or less, respectively. The area fractionsof polygonal ferrite and the martensite-austenite constituent may be 0%.

Average Grain Size of Bainite at Position of ½ Plate Thickness is 10 μmor Less

It is effective to form a fine microstructure for producing athick-walled steel plate having sufficiently high base metal toughnessparticularly at a position of ½ plate thickness. The above advantageouseffects may be produced by adjusting the grain size of bainite at aposition of ½ plate thickness to be 10 μm or less. Accordingly, theaverage grain size of bainite at a position of ½ plate thickness islimited to 10 μm or less.

The metal microstructure according to aspects of the present inventionmay include any phases other than bainite, polygonal ferrite, or themartensite-austenite constituent as long as it includes theabove-described structure. Examples of the other phases includepearlite, cementite, and martensite. The amount of the other phases ispreferably minimized; the area fraction of the other phases at aposition of ¼ plate thickness is preferably 5% or less.

In the steel material for line pipes according to aspects of the presentinvention, it is preferable that the ratio of the compressive strengthof the steel material to the tensile strength of the steel material be0.748 or more and the hardness of the steel material measured at aposition 1.5 mm from the surface of the steel pipe be HV 260 or less.Increasing the ratio of compressive strength to tensile strength andreducing the hardness of the surface layer enable a steel pipe havingsuitable roundness to be produced stably. It is more preferable that theratio of the compressive strength of the steel material to the tensilestrength of the steel material be 0.750 or more and the hardness of thesteel material measured at a position 1.5 mm from the surface of thesteel pipe be HV 256 or less.

3. Method for Producing Steel Material for Line Pipes

The method for producing a steel material for line pipes according toaspects of the present invention includes heating a steel slab havingthe above-described chemical composition, hot rolling the steel slab,subsequently performing accelerated cooling, and then performingtempering (reheating). The reasons for limiting the productionconditions are described below. Hereinafter, the term “temperature”refers to the average temperature of the steel plate (steel material) inthe thickness direction, unless otherwise specified. The averagetemperature of the steel plate (steel material) in the thicknessdirection is determined on the basis of thickness, surface temperature,cooling conditions, etc. by simulation calculation or the like. Forexample, the average temperature of the steel plate (steel material) inthe thickness direction may be calculated from a temperaturedistribution in the thickness direction determined by a finitedifference method.

Steel Slab Heating Temperature: 1000° C. to 1200° C.

If the steel slab heating temperature is less than 1000° C., NbC doesnot dissolve sufficiently and, consequently, precipitation strengtheningmay not be achieved in the subsequent step. On the other hand, if thesteel slab heating temperature is more than 1200° C., low-temperaturetoughness may become degraded. Accordingly, the steel slab heatingtemperature is limited to 1000° C. to 1200° C. Preferable lower limit ofthe steel slab heating temperature is 1000° C. and preferable upperlimit is 1150° C.

Cumulative Rolling Reduction Ratio in Non-Recrystallization TemperatureRange: 60% or More, and Cumulative Rolling Reduction Ratio inTemperature Range of (Rolling Finish Temperature +20° C.) or Less: 50%or More

For achieving high base metal toughness, it is necessary to performsufficient rolling reduction within the non-recrystallizationtemperature range in the hot rolling process. However, if the cumulativerolling reduction ratio in the non-recrystallization temperature rangeis less than 60% or the cumulative rolling reduction in the temperaturerange of (rolling finish temperature +20° C.) or less is less than 50%,the size of crystal grains may not be reduced to a sufficient degree.Accordingly, the cumulative rolling reduction ratio in thenon-recrystallization temperature range is limited to 60% or more, andthe cumulative rolling reduction in the temperature range of (rollingfinish temperature +20° C.) or less is limited to 50% or more. Thecumulative rolling reduction ratio in the non-recrystallizationtemperature range is preferably 65% or more. The cumulative rollingreduction ratio in the temperature range of (rolling finish temperature+20° C.) or less is preferably 55% or more.

Rolling Finish Temperature: Ar₃ Transformation Point or More and 790° C.or Less

For limiting the reduction in strength due to the Bauschinger effect, itis necessary to form a metal microstructure composed primarily ofbainite and suppress the formation of soft microstructures, such aspolygonal ferrite. This requires the hot rolling to be performed withinthe temperature range of the Ar₃ transformation point or more, in whichpolygonal ferrite does not form. Accordingly, the rolling finishtemperature is limited to the Ar₃ transformation point or more. Forachieving high base metal toughness, it is necessary to perform therolling at lower temperatures in the temperature range of the Ar₃transformation point or more. Accordingly, the upper limit for therolling finish temperature is set to 790° C. The rolling finishtemperature is preferably 780° C. or less.

Cooling Start Temperature: Ar₃ Transformation Point or More

If the cooling start temperature is less than the Ar₃ transformationpoint, the area fraction of polygonal ferrite at a position of 1/4 platethickness may exceed 10% and a sufficiently high compressive strengthmay not be achieved due to the Bauschinger effect. Accordingly, thecooling start temperature is limited to the Ar₃ transformation point ormore. The cooling start temperature is preferably (the Ar₃transformation point +10° C.) or more.

As described above, the Ar₃ transformation point can be calculated usingFormula (3).

Ar₃(° C.)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo   (3)

In Formula (3), the symbol of each element represents the content (mass%) of the element and is zero when the composition does not contain theelement.

Cooling Rate: 10° C./s or More

Accelerated cooling performed at a cooling rate of 10° C./s or more is aprocess essential for producing a high strength steel plate having hightoughness. Performing cooling at a high cooling rate enables strength tobe increased due to transformation strengthening. However, if thecooling rate is less than 10° C./s, a sufficiently high strength may notbe achieved. Furthermore, diffusion of C may occur. This results inconcentrating of C at non-transformed austenite and an increase in theamount of MA formed. Consequently, compressive strength may be reduced,because the presence of hard second phases, such as MA, accelerates theBauschinger effect as described above. When the cooling rate is 10° C./sor more, diffusion of C which occurs during the cooling may besuppressed and, consequently, the formation of MA may be reduced.Accordingly, the cooling rate in the accelerated cooling is limited to10° C./s or more. The cooling rate is preferably 20° C./s or more.

Cooling Stop Temperature: 200° C. to 450° C.

Performing rapid cooling to a temperature of 200° C. to 450° C. by theaccelerated cooling subsequent to the rolling enables the formation of abainite phase and a uniform microstructure. However, if the cooling stoptemperature is less than 200° C., an excessive amount of MA may beformed. This results in a reduction in compressive strength due to theBauschinger effect and degradation of toughness. On the other hand, ifthe cooling stop temperature is more than 450° C., pearlite may beformed. This makes it not possible to achieve a sufficiently highstrength and results in a reduction in compressive strength due to theBauschinger effect. Accordingly, the cooling stop temperature is limitedto 200° C. to 450° C. Preferable lower limit of the cooling steptemperature is 250° C. and preferable upper limit is 430° C.

Temperature of Surface of Steel Plate During Reheating: 350° C. to 550°C.

Reheating is performed subsequent to the accelerated cooling. In theaccelerated cooling of the steel plate, the cooling rate in thesurface-layer portion of the steel plate is high, and the surface-layerportion of the steel plate is cooled to a lower temperature than theinside of the steel plate. Consequently, the martensite-austeniteconstituent is likely to be formed in the surface-layer portion of thesteel plate. Since hard phases, such as MA, accelerate the Bauschingereffect, heating the surface-layer portion of the steel plate subsequentto the accelerated cooling to decompose MA may limit the reduction incompressive strength due to the Bauschinger effect. Furthermore, heatingthe surface-layer portion of the steel plate such that the temperatureof the surface of the steel plate reaches 350° C. or more may reduce thehardness of the surface-layer portion of the steel plate. However, ifthe temperature of the surface of the steel plate is less than 350° C.,the decomposition of MA may be insufficient. If the temperature of thesurface of the steel plate is more than 550° C., the temperature towhich the central portion of the steel plate is heated can also beincreased accordingly. This makes it difficult to achieve thepredetermined strength stably. Accordingly, the temperature of thesurface of the steel plate during reheating subsequent to theaccelerated cooling is limited to 350° C. to 550° C. The temperature ofthe surface of the steel plate is preferably 400° C. to 530° C.

Temperature of Center of Steel Plate During Reheating: Less Than 550° C.

Performing appropriate reheating subsequent to the accelerated coolingenables the decomposition of MA in the surface-layer portion and a highcompressive strength. Furthermore, setting the temperature to which thecentral portion of the steel plate is heated during reheating to be lessthan 550° C. may limit a reduction in strength due to the heating.However, if the temperature of the center of the steel plate is 550° C.or more, aggregation and coarsening of cementite may occur, whichdegrades low-temperature toughness. Moreover, it becomes difficult toachieve the predetermined strength stably. Accordingly, the temperatureof the center of the steel plate during reheating subsequent to theaccelerated cooling is limited to be less than 550° C.

Examples of means for performing reheating subsequent to the acceleratedcooling include, but are not limited to, atmosphere furnace heating, gascombustion, and induction heating. Induction heating is preferable inconsideration of economy, controllability, etc.

4. Method for Producing Line Pipe

In accordance with aspects of the present invention, a steel pipe (linepipe) is produced using a steel plate (steel material) produced by theabove-described method. Examples of a method for forming the steelmaterial include a method in which a steel material is formed into theshape of a steel pipe by cold forming, such as a UOE process or pressbending (also referred to as “bending press”). In the UOE process, theedges of a steel plate (steel material) in the width direction aresubjected to edge preparation and then the edge of the steel plate inthe width direction is crimped using a C-press machine. Subsequently,the steel plate is formed into a cylindrical shape such that the edgesof the steel plate in the width direction face each other using a U-ingpress machine and an O-ing press machine. Then, the edges of the steelplate in the width direction are brought into abutment with and weldedto each other. This welding is referred to as “seam welding”. The seamwelding is preferably performed using a method including two steps, thatis, a tack welding step of holding the cylindrical steel plate, bringingthe edges of the steel plate in the width direction into abutment witheach other, and performing tack welding; and a final welding step ofsubjecting the inner and outer surfaces of the seam of the steel plateto welding using a submerged arc welding method. After the seam welding,pipe expansion is performed in order to remove welding residual stressand to improve the roundness of the steel pipe. In the pipe expansionstep, the expansion ratio (the ratio of a change in the outer diameterof the pipe which occurs during the pipe expansion to the outer diameterof the pipe before the pipe expansion) is set to 1.2% or less. This isbecause, if the expansion ratio is excessively high, compressivestrength may be significantly reduced due to the Bauschinger effect. Theexpansion ratio is preferably 1.0% or less. The expansion ratio ispreferably 0.4% or more and is more preferably 0.6% or more in order toreduce welding residual stress and enhance the roundness of the steelpipe.

In the press bending, the steel plate is repeatedly subjected tothree-point bending to gradually change its shape and, thereby, a steelpipe having a substantially circular cross section is produced. Then,seam welding is performed as in the UOE process described above. Also inthe press bending, pipe expansion may be performed after the seamwelding.

EXAMPLES

Slabs were manufactured from steels (Steel types A to K) having thechemical compositions described in Table 1 by a continuous castingprocess. Steel plates (Nos. 1 to 26) having a thickness of 35 to 40 mmwere manufactured from the slabs. Steel pipes were manufactured from thesteel plates by the UOE process. Seam welding was performed by four-wiresubmerged arc welding such that one welding path is formed on both ofthe inner and outer surfaces of the seam. The heat input during thewelding was selected from the range of 20 to 80 kJ/cm in accordance withthe thickness of the steel plate. Table 2 summarizes the conditionsunder which the steel plates were produced and the condition under whichthe steel pipes were produced (expansion ratio).

TABLE 1 Ar₃ Steel Composition (mass %) Ceq Pcm transformation type C SiMn Nb Ti Al Cu Ni Cr Mo V value ⁽¹⁾ value ⁽²⁾ point ⁽³⁾ Remark A 0.0500.150 1.80 0.028 0.012 0.030 0.020 0.200 0.250 0.100 0.020 0.439 0.171727 Invention B 0.060 0.230 1.75 0.020 0.015 0.033 0.200 0.365 0.159 740example C 0.095 0.040 1.55 0.025 0.010 0.020 0.200 0.393 0.187 741 D0.065 0.050 1.60 0.030 0.011 0.025 0.150 0.150 0.200 0.030 0.398 0.170748 E 0.060 0.040 1.90 0.025 0.020 0.033 0.050 0.300 0.050 0.410 0.167718 F 0.050 0.050 1.85 0.028 0.010 0.030 0.150 0.150 0.300 0.020 0.4420.171 731 G 0.025 0.100 1.58 0.028 0.020 0.025 0.600 0.005 0.329 0.118743 Comparative H 0.080 0.150 1.80 0.030 0.013 0.033 0.200 0.250 0.2000.200 0.490 0.213 704 example I 0.055 0.210 1.53 0.012 0.032 0.180 0.1500.352 0.155 765 J 0.140 0.150 1.55 0.025 0.011 0.028 0.200 0.200 0.4250.236 728 K 0.065 0.350 1.80 0.030 0.015 0.025 0.200 0.200 0.100 0.0100.414 0.188 723 *The underlined values are outside the scope of thepresent invention. ⁽¹⁾ Ceq = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5⁽²⁾ Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 ⁽³⁾Ar₃ = 910 − 310 C − 80 Mn − 20 Cu − 15 Cr − 55 Ni − 80 Mo

TABLE B Cumulative rolling reduction ratio Below (rolling Non- finishRolling Cooling Cooling Reheating Ar₃ Heating recrystallizationtemperature + finish start Cooling stop temperature Expansion Steeltransformation Thickness temperature temperature 20° C.) temperaturetemperature rate temperature (° C.) ratio No. type point (° C.) (mm) (°C.) range (%) (%) (° C.) (° C.) (° C./s) (° C.) Reheating facilitySurface Center (%) Remark 1 A 727 40 1050 75 70 760 755 25 280 Inductionheating furnace 470 420 0.8 Invention 2 A 727 40 1030 75 55 765 750 20300 Induction heating furnace 450 400 0.8 example 3 A 727 40 1040 75 75780 770 20 320 Induction heating furnace 450 400 0.8 4 A 727 40 1060 7570 765 755 27 280 Induction heating furnace 360 320 0.8 5 A 727 40 105075 70 760 750 22 310 Induction heating furnace 520 470 0.8 6 B 740 351100 80 75 775 770 30 260 Gas combustion furnace 460 450 0.8 7 C 741 351060 75 75 775 765 35 420 Induction heating furnace 520 470 1.0 8 D 74835 1100 75 70 770 760 30 300 Induction heating furnace 500 450 1.0 9 E718 35 1050 75 70 775 765 28 280 Induction heating furnace 470 420 1.010 F 731 40 1050 75 70 755 745 32 260 Induction heating furnace 460 4200.6 11 A 727 40  950 75 70 770 760 20 280 Induction heating furnace 460420 0.8 Comparative 12 A 727 40 1250 75 70 765 760 25 290 Inductionheating furnace 470 450 0.8 example 13 A 727 40 1050 55 55 760 750 25270 Induction heating furnace 450 400 0.8 14 A 727 40 1040 75 45 765 76025 310 Induction heating furnace 460 410 0.8 15 A 727 40 1030 75 70 725720 20 320 Induction heating furnace 450 400 0.8 16 A 727 40 1050 75 70800 790 30 300 Induction heating furnace 480 420 0.8 17 F 731 40 1070 7575 760 750 20 500 Induction heating furnace 540 510 0.8 18 F 731 40 104075 70 770 760 30 280 Induction heating furnace 320 280 0.8 19 F 731 401050 75 70 760 750 25 290 Induction heating furnace 600 550 0.8 20 A 72740 1050 75 70 760 755 25 280 None 0.8 21 F 731 40 1040 75 75 760 750 26290 Induction heating furnace 460 410 1.6 22 G 743 35 1060 75 70 775 77025 310 Induction heating furnace 480 430 0.8 23 H 704 40 1030 75 70 760750 20 320 Induction heating furnace 450 400 1.0 24 I 765 35 1050 80 75785 770 20 280 Induction heating furnace 420 380 0.8 25 J 728 40 1080 7575 765 755 25 310 Induction heating furnace 490 460 0.8 26 K 723 35 103075 70 770 760 25 290 Induction heating furnace 470 420 0.8 *Theunderlined values are outside the scope of the present invention.

To determine the tensile properties of the steel pipes produced asdescribed above, a full-thickness test piece in the circumferentialdirection of the pipe was taken from each of the steel pipes as a testpiece for tensile test and the tensile strength of the test piece wasmeasured by a tensile test. In a compression test, a test piece having adiameter of 20 mm and a length of 60 mm was taken from the innersurface-side portion of each of the steel pipes in the circumferentialdirection of the pipe and the 0.5% compressive proof strength of thetest piece was measured as a compressive yield strength.

A DWTT test piece was taken from each of the steel pipes in thecircumferential direction of the pipe. Using the DWTT test piece, thetemperature at which the percent ductile fracture reached 85% wasdetermined as 85% SATT.

For determining the HAZ toughness of the joint, the temperature at whichthe percent ductile fracture reached 50% was determined as vTrs. Theposition of the notch was determined such that the fusion line waslocated at the center of the notch root of the Charpy test piece and theratio between the weld metal and the base metal (including weldheat-affected zone) at the notch root was 1:1.

For determining the hardness of each of the steel pipes at a position1.5 mm from the surface, the hardness of the steel pipe was measured atrandomly selected 20 positions spaced at intervals of 10 mm in thecircumferential direction of the steel pipe at a depth of 1.5 mm belowthe inner surface of the steel pipe using a Vickers hardness tester witha load of 10 kgf (98 N) and the average thereof was calculated.

For determining metal microstructure, a sample was taken from the innersurface-side portion of each of the steel pipes at a position of ¼ platethickness. The sample was etched using nital after polishing, and themetal microstructure was observed using an optical microscope. The areafractions of bainite and polygonal ferrite were calculated by imageanalysis of 3 photographs captured at a 200-fold magnification. Forobserving MA, the sample used for measuring the area fractions ofbainite and polygonal ferrite was subjected to nital etching and thenelectrolytic etching (two-step etching). Subsequently, the metalmicrostructure was observed with a scanning electron microscope (SEM).The area fraction of MA was calculated by image analysis of 3photographs captured at a 1000-fold magnification. The average grainsize of bainite was determined by a linear analysis using a micrographobtained by taking a sample from the inner surface-side portion of eachof the steel pipes at a position of ¼ plate thickness, etching thesample using nital after polishing, and observing the metalmicrostructure using an optical microscope.

Although the metal microstructures of the steel pipes are determined inExamples, the results may be considered as the metal microstructures ofthe respective steel plates (steel materials).

Table 3 shows the metal microstructures and mechanical propertiesmeasured.

TABLE 3 Metal microstructure Plate thickness 1/4 position AreaMechanical properties Area fraction of Plate thickness HV (10 kg) atDWTT HAZ Area fraction of martensite- 1/2 position Tensile CompressiveCompressive position 1.5 mm property toughness Steel fraction ofpolygonal austenite Bainite grain strength strength strength/tensilefrom steel 85% vTrs No. type bainite (%) ferrite (%) constituent (%)Balance size (μm) (MPa) (MPa) strength pipe surface SATT (° C.) (° C.)Remark 1 A 94.5 3.7 0.9 θ 6.5 620 511 0.824 228 −27 −35 Invention 2 A93.8 4.5 1.3 θ 8.5 612 519 0.848 230 −22 −37 example 3 A 98.3 0.0 1.5 θ9.2 634 563 0.888 235 −17 −35 4 A 94.9 1.7 3.2 θ 6.0 648 487 0.752 254−30 −37 5 A 94.8 3.5 0.5 θ 7.3 603 476 0.789 214 −26 −35 6 B 91.6 4.83.2 θ 7.5 586 469 0.800 216 −25 −50 7 C 91.2 4.5 4.1 θ 6.5 591 538 0.910212 −33 −25 8 D 89.5 8.0 1.3 θ 7.2 613 471 0.769 216 −28 −27 9 E 98.00.0 1.4 θ 6.5 610 486 0.796 226 −22 −40 10 F 95.5 2.3 1.1 θ 7.0 620 5620.906 225 −23 −37 11 A 98.5 0.0 1.2 θ 6.0 562 470 0.836 210 −32 −38Comparative 12 A 97.5 0.0 1.7 θ 18.0 732 633 0.864 256 −5 −38 example 13A 95.1 2.3 1.5 θ 17.0 622 503 0.809 226 −5 −37 14 A 98.0 0.0 1.1 θ 17.5623 549 0.882 240 0 −36 15 A 74.5 22.0 2.5 θ 6.0 565 425 0.752 209 −32−37 16 A 97.9 0.0 1.5 θ 20.0 646 554 0.858 255 0 −37 17 F 93.2 2.0 1.3θ, P 9.0 580 429 0.740 205 −15 −39 18 F 93.2 0.0 6.8 — 6.5 672 436 0.648266 −25 −39 19 F 96.3 0.0 1.4 θ 7.5 582 369 0.634 212 −17 −41 20 A 86.46.0 7.6 — 8.2 669 376 0.561 252 −17 −41 21 F 96.5 2.0 1.2 θ 6.5 619 4300.695 228 −25 −41 22 G 96.9 2.5 0.3 θ 7.0 526 434 0.825 197 −20 −53 23 H95.6 0.0 2.3 θ, P 6.0 673 582 0.865 250 −33 −5 24 I 86.0 11.0 2.1 θ 8.7561 417 0.744 208 −14 −42 25 J 90.8 1.5 6.1 θ, P 6.2 614 439 0.714 226−25 0 26 K 94.3 0.0 5.7 — 6.5 600 437 0.728 223 −23 −28 * The underlinedvalues are outside the scope of the present invention. * In the abovetable, ″θ″ and ″P″ denote cementite and pearlite, respectively.

In Table 3, all of Nos. 1 to 10 had a tensile strength of 570 MPa ormore; a compressive strength of 440 MPa or more; as for DWTT property, a85% SATT of −10° C. or less; and a HAZ toughness of −20° C. or less.That is, all of Nos. 1 to 10 were evaluated as good. Moreover, in all ofNos. 1 to 10, the ratio of compressive strength to tensile strength was0.75 or more and hardness at a position 1.5 mm from the surface of thesteel pipe was HV 260 or less. This is effective for producing steelpipes having suitable roundness further stably.

In contrast, in Nos. 11 to 21, although the composition fell within thescope according to aspects of the present invention, the productionmethod was outside the scope of the present invention and therefore theintended microstructure was not formed. As a result, Nos. 11 to 21 wereevaluated as poor in terms of any of tensile strength, compressivestrength, and DWTT property. In Nos. 22 to 26, the chemical compositionwas outside the scope of the present invention. As a result, Nos. 22 to26 were evaluated as poor in terms of any of tensile strength,compressive strength, DWTT property, and HAZ toughness. In Nos. 18 and19, the production conditions during reheating were outside the scope ofthe present invention. As a result, Nos. 18 and 19 were evaluated aspoor in terms of the ratio of compressive strength to tensile strengthand hardness at a position 1.5 mm from the surface of the steel pipe.

According to aspects of the present invention, a high-strength steelpipe of API-X70 grade or more which has excellent low-temperaturetoughness and an excellent DWTT property may be produced. Therefore, thesteel pipe according to aspects of the present invention may be used asdeep-sea line pipes that require high collapse resistant performance.

1. A method for producing a steel material for line pipes, the steelmaterial having a tensile strength of 570 MPa or more, a compressivestrength of 440 MPa or more, and a thickness of 30 mm or more, themethod comprising heating a steel having a composition containing, bymass, C: 0.030% to 0.10%, Si: 0.01% to 0.30%, Mn: 1.0% to 2.0%, Nb:0.005% to 0.050%, Ti: 0.005% to 0.025%, and Al: 0.08% or less, thecomposition further containing one or more elements selected from, bymass, Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0.5% orless, and V: 0.1% or less, wherein a Ceq value represented by Formula(1) is 0.350 or more, a Pcm value represented by Formula (2) is 0.20 orless, and an Ar₃ transformation point represented by Formula (3) is 750°C. or less, with the balance being Fe and inevitable impurities, to atemperature of 1000° C. to 1200° C.; performing hot rolling such that acumulative rolling reduction ratio in a non-recrystallizationtemperature range is 60% or more, such that a cumulative rollingreduction ratio in a temperature range of (a rolling finish temperature+20° C.) or less is 50% or more, and such that a rolling finishtemperature is the Ar₃ transformation point or more and 790° C. or less,the rolling finish temperature being an average temperature of a steelplate; subsequently performing accelerated cooling from a temperature ofthe Ar₃ transformation point or more, at a cooling rate of 10° C./s ormore, to a cooling stop temperature of 200° C. to 450° C., the coolingstop temperature being an average temperature of the steel plate; andthen performing reheating such that the temperature of a surface of thesteel plate is 350° C. to 550° C. and such that the temperature of thecenter of the steel plate is less than 550° C.,Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5   (1)Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10   (2)Ar₃(° C.)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo   (3) wherein, in Formulae(1) to (3), the symbol of each element represents the content (mass %)of the element and is zero when the composition does not contain theelement.
 2. A method for producing a line pipe having a tensile strengthof 570 MPa or more, a compressive strength of 440 MPa or more, and athickness of 30 mm or more, the method comprising cold forming a steelmaterial for line pipes produced by the method according to claim 1 intoa steel pipe-like shape; joining butting edges to each other by seamwelding; and subsequently performing pipe expansion at an expansionratio of 1.2% or less to produce a steel pipe.
 3. A steel material forline pipes, the steel material having a tensile strength of 570 MPa ormore, a compressive strength of 440 MPa or more, and a thickness of 30mm or more, the steel material comprising a composition containing, bymass, C: 0.030% to 0.10%, Si: 0.01% to 0.30%, Mn: 1.0% to 2.0%, Nb:0.005% to 0.050%, Ti: 0.005% to 0.025%, and Al: 0.08% or less, thecomposition further containing one or more elements selected from, bymass, Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0.5% orless, and V: 0.1% or less, wherein a Ceq value represented by Formula(1) is 0.350 or more, a Pcm value represented by Formula (2) is 0.20 orless, and a Ar₃ transformation point represented by Formula (3) is 750°C. or less, with the balance being Fe and inevitable impurities, thesteel material further comprising a metal microstructure composedprimarily of bainite, wherein an area fraction of polygonal ferrite at aposition of ¼ plate thickness is 10% or less, an area fraction ofmartensite-austenite constituent at the position of ¼ plate thickness is5% or less, and an average grain size of bainite at a position of ½plate thickness is 10 μm or less,Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5   (1)Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10   (2)Ar₃(° C.)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo   (3) wherein, in Formulae(1) to (3), the symbol of each element represents the content (mass %)of the element and is zero when the composition does not contain theelement.
 4. The steel material for line pipes according to claim 3,wherein a ratio of compressive strength to tensile strength is 0.748 ormore, and wherein a hardness measured at a position 1.5 mm from an innersurface of a steel pipe is HV 260 or less.
 5. A method for producing aline pipe having a tensile strength of 570 MPa or more, a compressivestrength of 440 MPa or more, and a thickness of 30 mm or more, themethod comprising cold forming a steel material for line pipes accordingto claim 3 into a steel pipe-like shape; joining butting edges to eachother by seam welding; and subsequently performing pipe expansion at anexpansion ratio of 1.2% or less to produce a steel pipe.
 6. A method forproducing a line pipe having a tensile strength of 570 MPa or more, acompressive strength of 440 MPa or more, and a thickness of 30 mm ormore, the method comprising cold forming a steel material for line pipesaccording to claim 4 into a steel pipe-like shape; joining butting edgesto each other by seam welding; and subsequently performing pipeexpansion at an expansion ratio of 1.2% or less to produce a steel pipe.